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Branton Campbell in the lab

Branton J. Campbell

Professor

Department of Physics & Astronomy

Brigham Young University

Provo, UT 84602, USA

Tel: 801-422-5758, Fax: 801-422-0553

Email: branton_campbell[at]byu.edu

//physics.byu.edu/faculty/campbell/

Research Interests

I apply state-of-the-art x-ray and neutron scattering techniques to study local and long-range structures in a variety of complex solids, including fast-ion conductors, ferroelectric relaxors, high-temperature superconductors, and colossal magnetoresistive manganites, where nanoscale structural features influence macroscopic physical properties. This includes the development of symmetry-mode analysis (through the tools of the ISOTROPY Software Suite) for the determination, refinement and interpretation of distorted structures involving lattice strains, atomic displacements, magnetic moments and occupational orderings at both commensurate and incommensurate wavevectors.

A Big Table of Space-Group IRs

The irreducible representations (IRs) of the parent symmetry of a system provide a symmetry-motivated parameter set for describing any periodic or aperiodic distortion. The IRs of complete crystallographic space groups and their extensions to (3+d)-dimensional superspace, including all special and non-special k-vectors, commensurate and incommensurate, have now been exhaustively tabulated for the first time. Photo (Wikipedia): Table Mountain, Capetown, South Africa.

Selected Publications

ISODISPLACE: a web-based tool for exploring structural distortions

Branton J. Campbell, Harold T. Stokes, David E. Tanner, and Dorian M. Hatch

ISODISPLACE is a new internet-server tool for exploring structural phase transitions. Given parent-phase structural information, it generates atomic displacement patterns induced by irreducible representations of the parent space-group symmetry and allows a user to visualize and manipulate the amplitude of each distortion mode interactively. ISODISPLACE is freely accessible at http://stokes.byu.edu/isodisplace.html via common internet browsers.

Diffuse scattering in the layered perovskites

B. J. Campbell

Single-crystal X-ray and neutron diffuse scattering techniques have been used to probe a variety of short-range displacive and magnetic orderings in the layered transition-metal perovskites which arise due to the intimate coupling of their structural, electronic and magnetic degrees of freedom. The present work reviews recent analyses that have successfully modeled short-range intralayer and/or interlayer correlations in these materials leading to useful physical insights, and describes the methods employed within a general formulation of the disordered structure function.

Structure, thermal expansion and transport properties

Li-Ping Li and Branton J. Campbell (et al.)

Perovskite oxides BaCe1-xEuxO3-delta (x = 0.05, 0.10, 0.15, and 0.20) were prepared using starting ratios of Ba/(Ce + Eu) = 1.0 and 1.1. X-ray diffraction analysis shows that preparations with a slight initial barium excess crystallized as a single perovskite phase, while the stoichiometric preparations crystallized with a trace of CeO2 due to Ba loss. EPR measurements indicate that the samples have no Ba vacancies and that Eu and Ce ions are in trivalent and tetravalent states, respectively. The single-phase samples synthesized from Ba/(Ce + Eu) = 1.1 preparations are shown to have different cell volume trends versus x, lower average thermal expansion coefficients, and higher proton conductivities in water-containing atmospheres between 80 and 700 degreesC, compared to samples synthesized from the Ba/(Ce + Eu) = 1.0 preparations. These observations are explained in terms of the impurity CeO2) phase, Eu doping-site preferences, and oxygen vacancy generation.

Elucidation of zeolite microstructure by synchrotron X-ray diffuse scattering

B. J. Campbell (et al.)

Single-crystal diffuse scattering measurements can now rapidly probe the three-dimensional structure of subtle defects in microporous framework materials. Diffuse scattering data from natural mordenite crystals are shown to exhibit a complex distribution of weak features which have been mapped out using a synchrotron X-ray source and a CCD detector. Comparison with computer-simulated diffuse scattering patterns yields a detailed three-dimensional columnar defect structure and reveals that roughly one third of the mordenite's columnar defects cooperate to form a block-mosaic pattern of {110} stacking faults.

Enhanced stability of charge and orbital order in La_0.78Sr_2.22Mn₂O₇

B. J. Campbell (et al.)

Not only is zigzag charge and orbital (CO) order shown to be present in the highly doped (x=0.61) bilayered manganite system La2-2xSr1+2xMn2O7, it demonstrates an enhanced stability relative to the x=0.5 composition where the ordered state might be expected to have the lowest entropy. No magnetic-field dependence is observed up to 5.25 T. The CO ordering temperature also increases from 210 K at x=0.5 to 230 K, and the broad hysteretic conversion of the CO-ordered state to the A-type AF state at low temperatures is only partial. Phase reorganization in this hysteretic mixed-phase region is seen to involve an exponential time relaxation above T=50 K, while reorganization ceases below 50 K, apparently due to the thermally activated nature of the underlying dynamics.